1. Technical Field
The present invention relates to electrochemical fuel cells, and in particular, to methods of operating fuel cell stacks and systems to mitigate sulfur contamination.
2. Description of the Related Art
Fuel cells convert fuel and oxidant to electricity and reaction product. Proton exchange membrane fuel cells employ a membrane electrode assembly (“MEA”) having a proton exchange membrane (“PEM”) (also known as an ion-exchange membrane) interposed between an anode electrode and a cathode electrode. The anode electrode typically includes electrocatalyst and binder, often a dispersion of polytetrafluoroethylene (PTFE) or other hydrophobic polymer, and may also include a filler (e.g., carbon). The anode electrode may also comprise electrocatalyst and an ionomer, or a mixture of electrocatalyst, ionomer and binder. The presence of ionomer in the electrocatalyst layer effectively increases the electrochemically active surface area of the electrocatalyst, which requires an ionically conductive pathway to the cathode electrocatalyst to generate electric current. The cathode electrode may similarly include electrocatalyst and binder and/or ionomer. Typically, the electrocatalyst used in the anode and the cathode is platinum or platinum alloy (e.g., platinum black, platinum-rutheninum and platinum-cobalt, and others commonly known in the art). The electrocatalyst may or may not be supported on an electrically-conductive support material, such as carbon black, graphitized carbon, or graphite. Each electrode may further include a microporous, electrically conductive substrate, such as carbon fiber paper or carbon cloth, which provides structural support to the membrane and serves as a fluid diffusion layer. The anode and cathode electrodes may be bonded or sealed to the PEM to form a single integral MEA unit.
The MEA is further interposed between two fluid flow plates to form a fuel cell assembly. The plates allow access of reactants to the MEA, act as current collectors, and provide support for the adjacent electrodes. A plurality of fuel cell assemblies may be combined to form a fuel cell stack.
During fuel cell operation, a primary load is drawn from the fuel cell. At the anode, fuel (typically in the form of hydrogen gas) reacts at the anode electrocatalyst in the presence of the PEM to form hydrogen ions and electrons. At the cathode, oxidant (typically oxygen in air) reacts with the hydrogen ions, which pass through the PEM, in the presence of the cathode electrocatalyst to form water. The PEM also serves to isolate the fuel stream from the oxidant stream while facilitating the migration of the hydrogen ions from the anode to the cathode. The electrons pass through an external circuit, creating a flow of electricity to sustain the primary load. In practice, fuel cells need to be robust to varying operating conditions and impurities in the reactants that poison or contaminate the fuel cell electrocatalyst.
Reformed fuels are typically employed in fuel cells and fuel cell systems that employ hydrocarbon-based fuels, such as natural gas and gasoline. Such fuels can be stored on-board fuel cell systems or provided via gas lines, and reformed in a reformer to produce hydrogen and carbon dioxide along with small amounts of impurities, such as carbon monoxide and hydrogen sulfide (H2S). It is known in the art that hydrogen sulfide poisons platinum-based anode electrocatalysts by adsorbing onto the electrocatalyst as Pt—(H2S)ads, thereby decreasing the effective platinum surface area (EPSA) and, thus, fuel cell performance. As the contamination mechanism is cumulative, a continuous supply of the reformed fuel will lead to increasing amounts of hydrogen sulfide adsorbing on the electrocatalyst. It is generally known that hydrogen sulfide can be removed from the anode platinum-based electrocatalyst by cyclic voltammetry scanning between 0 and 1.4 V versus DHE (dynamic hydrogen electrode), or by applying a high voltage pulse (1.5 V for 2 min) followed by a low voltage pulse (0.2 V for 2 min) to each cell (see W. Shi et al., Journal of Power Sources, 165: 814-818, 2007). In other methods, an adsorption means may be employed upstream of the fuel cell stack to remove hydrogen sulfide from the reformed fuel prior to supplying the fuel to the fuel cell stack.
Impurities in the air used for the oxidant can also have a negative effect on fuel cell performance. One impurity typically found in air is sulfur dioxide (SO2). Sulfur dioxide adsorbs onto the platinum electrocatalyst in the cathode as Pt—(SO2)ads in a cumulative fashion, and continually decreases fuel cell performance as air (and sulfur dioxide) is continuously supplied. In polluted air, sulfur dioxide poisoning becomes an even more significant problem because sulfur dioxide can exist in large concentrations, sometimes exceeding 0.125 ppm (parts per million) in highly polluted air. It is generally known that fuel cell performance can be at least partially recovered from such poisoning by cyclic voltammetry scanning, for example, between 0 and 0.9 V versus DHE, followed by high humidity operation (see Y. Nagahara et al., Journal of Power Sources, 182: 422-428, 2008). Sulfur dioxide may also be decreased or prevented from entering the fuel cell by employing a filter means upstream of the fuel cell.
In summary, there are many methods to remove poisons and impurities that adsorb onto the platinum electrocatalyst. However, such methods are often difficult to employ in real-world applications. Potential cycling performed by cyclic voltammetry scanning is typically not used in commercial fuel cell systems as it requires additional equipment that is not commonly found in commercial fuel cell systems, and thus is not practical. In addition, filters and adsorption means are not preferable due to increased cost and/or increased system complexity, and trace amounts of sulfur may still be introduced into the fuel cell system. As a result, there remains a need for improved methods to mitigate sulfur poisoning in fuel cells. The present invention addresses these needs and provides further related advantages.